The invention relates to sulphonated polymer resins, particularly ion-exchange resins, and the preparation of such resins. The invention relates especially to polymer resins whose shell layer is sulphonated, and to the preparation thereof. The polymer material to be sulphonated is typically for example a cross-linked styrene-divinylbenzene copolymer (a styrene copolymer cross-linked with divinylbenzene). The obtained sulphonated polymer resins are useful for example as chromatographic resins, ion-exchange resins and catalyst resins, either in a spherical or pulverized form.
Polystyrene-based resins are conventionally sulphonated for example by concentrated sulphuric acid in a swelling agent (usually a chlorohydrocarbon). However, the use of chlorohydrocarbons has been reduced for example due to environmental reasons. Polystyrene-based resins have also been sulphonated directly by a gaseous sulphur trioxide. It has been noted, however, that the obtained products where polymer particles are fully sulphonated are not physically stable but they tend to break.
European Published Application 0,361,685 (Rohm and Haas Co.) describes partly functionalized, for example sulphonated, polymer resin particles and a process for producing them. The process provides polymer resin particles where 68 to 98% of the accessible functionalizable sites are functionalized (e.g. sulphonated). The unfunctionalized sites are situated in the inner core of the particles, whereas the functionalized groups are located in the shell layer of the particles. The depth of functionalization can be for example 0.32 to 0.75 times the average radius of the polymer particles. In this publication the sulphonation is carried out with concentrated sulphuric acid at a normal pressure at a high temperature, such as 120 to 140xc2x0 C.
U.S. Pat. No. 3,252,921 (Dow Chemical Company) describes partial/heterogeneous sulphonation of alkenylaromatic polymer resins by first using a swelling agent (a chlorohydrocarbon) and by thereafter carrying out the actual sulphonation with chlorosulphonic acid or liquid sulphur trioxide. This provides polymer particles with a sulphonated shell layer.
Example 3 of German Offenlegungsschrift 2,627,877 (Sumitomo Chemical Co.) describes sulphonation of fibrous polyethylene with gaseous sulphur trioxide in a vacuum. According to claim 4, the process is carried out at a low temperature (10 to 90xc2x0 C.). The degree of sulphonation may vary within a broad range, such as 0.01 to 10 meq/g. The sulphonation reagent is said to have a concentration preferably in a range of from 10 to 80% by volume of SO3. If the SO3 content of the sulphonation gas is higher, the sulphonation reaction does not proceed in a uniform manner.
French Patent 1,280,353 (Rohm and Haas Co.) describes the sulphonation of macroporous vinylaromatic polymers with gaseous sulphur trioxide (usually in a mixture with air) at a normal pressure. According to the examples, the temperature varies from 60 to 100xc2x0 C. The publication does not disclose the preparation of partly sulphonated products.
The extent to which resins are sulphonated is indicated by their degree of sulphonation, which is usually given in dry weight capacity. The theoretical dry weight capacity (one sulphone group per benzene ring) of a monosulphonated styrene-divinylbenzene copolymer resin varies between 4.8 and 5.4 meq/g.
One of the essential properties of ion-exchange resins is their capacity. The capacity of a sulphonated styrene-divinylbenzene copolymer resin indicates how many H+ ions it can exchange per one mass and/or volumetric unit of resin. The capacity can be given as dry weight capacity or volume capacity. The dry weight capacity is indicated as milliequivalents per one gram of dry resin (meq/g) and the volume capacity is indicated as equivalents per one litre of fully swollen resin (eq/l).
When a resin is transferred from one medium to another, it may either swell or shrink. Great changes in the volume hinder the use of the resin in columns, wherefore the variation in volume should be minimal.
Resin particle size and the distribution thereof essentially affect the behaviour of an ion-exchange resin, such as kinetics of mass transfer, pressure drop over a backed bed, flow channelling and the degree of packing of the bed. The mean particle size of resin or the mean sphere size (resin particles are usually spherical) refers to an average based on the volume or mass proportion of different size fractions. The sharpness of the sphere size distribution is generally described by means of a uniformity coefficient (UC). This coefficient is calculated by forming a quotient between a mesh size that retains 40% of resin particles and a mesh size retaining 90% of resin particles. This ratio is given value 1 when all the particles are of equal size. For example, a typical resin intended for water treatment has a uniformity coefficient UC=1.7. The UC of industrial chromatographic separation resins varies between 1.05 and 1.25.
The degree of cross-linking of the sulphonated styrene-divinylbenzene copolymer resin is dependent on the amount of the divinylbenzene used as a cross-linking agent during the polymerization. A gel-type resin normally comprises 1 to 12% of divinylbenzene. The degree of cross-linking affects for example the mechanical strength, ion exchange capacity, water retention capacity, swelling, selectivity and chemical stability of the ion exchanger. Resins with a low degree of cross-linking are soft and mechanically unstable, whereas a high degree of cross-linking provides hardness, fragility and increased sensitivity to osmotic effects.
There are two main types of ion-exchange resins (e.g. sulphonated styrene-divinylbenzene copolymer resins): gel-type and macroporous resins. A macroporous ion-exchange resin is a resin where additional blowing agent has been added to the monomer mixture during polymerization and removed thereafter. This provides a structure with far greater pores than in the polymer network. A gel-type ion-exchange resin, in turn, refers to a resin where the porosity is only based on the porosity of the cross-linked polymer network.
Mechanical strength describes the resin""s ability to resist wearing. In a physically advantageous ion-exchange resin the particles are spherical in shape, and the resin does not comprise cracks and is not fragile. Mechanical strength is examined for example by a cyclic test of watering and drying, where the resin strength is examined by subjecting the resin to repeated watering and drying operations. Physical hardness is measured by means of compression resistance. The resistance of a resin to osmotic forces is important in industrial applications. Several methods have been introduced to measure the resistance of a resin to osmotic shock.
Chemical stability of a resin refers to the resistance of active groups and the hydrocarbon backbone particularly to oxidation.
The primary factor restricting the use of a strongly acidic cation-exchange resin at high temperatures is desulphonation. A typical maximum operating temperature of such resins in long-term use is 120xc2x0 C.
In the present invention the sulphonation of the shell layer means that the polymer particles are not fully sulphonated but the sulphonation is only effected beginning from the surface of the particles so that the core remains unsulphonated. There is a clearly defined interface between the sulphonated and unsulphonated regions. The sulphonation depth can vary.
Sulphonation with xe2x80x98substantially pure gaseous sulphur trioxidexe2x80x99 means in the present invention that the space where the sulphonation is carried out is substantially free of diluting gas components, such as air.
The object of the invention is to provide a process for preparing a sulphonated stable polymer resin so that the sulphonation can be carried out without a swelling agent, such as a chlorohydrocarbon. This problem has been solved in the invention such that the sulphonation of a polymer resin in a non-swollen state is performed by substantially pure gaseous sulphur trioxide. Sulphonation is substantially performed on the shell layer of polymer particles.
The objects of the invention are achieved by means of a product and a process which are characterized by what is disclosed in independent claims 1 and 21. The preferred embodiments of the invention are disclosed in the dependent claims.
The invention also relates to the use of the obtained sulphonated resin as a chromatographic resin, ion-exchange resin and catalyst resin.
The invention relates to a sulphonated polymer resin, which is characterized in that it is prepared by sulphonating a non-swollen polymer with substantially pure gaseous sulphur trioxide substantially in the shell layer of the polymer particles.
The invention also relates to a process for preparing a sulphonated polymer resin. The process is characterized by sulphonating a polymer in a non-swollen state with substantially pure gaseous sulphur trioxide substantially in the shell layer of the polymer particles.
The polymer used as a starting material is sulphonated substantially in the shell layer of the polymer particles. The sulphonation depth and the degree of sulphonation indicating the depth can vary. The thickness of the sulphonated shell layer can vary for example between 1 and 80%, preferably 10 and 50%, calculated from the average radius of a polymer particle.
In a preferred embodiment the interface between the sulphonated and the unsulphonated regions is clearly defined in the polymer resin particles according to the invention whose shell layer is sulphonated.
A polymer is preferably sulphonated at a reduced pressure, and the reaction space containing the polymer is subjected to a reduced pressure already before the sulphonation in order to remove diluting gas components, such as air. The pressure produced in the reaction space is typically lower than 10000 Pa, preferably less than 1000 Pa, and most preferably between 50 and 100 Pa.
The polymer used as a starting material is typically other than polyethylene. The polymer is typically an alkenylaromatic polymer, which is preferably cross-linked. Typical alkenylaromatic polymers include vinylaromatic polymers. Examples of vinyl aromatic- monomers are styrene, methylstyrene, ethylstyrene, etc. and other styrenic derivatives.
An advantageous polymer is a polymer with a styrene skeleton. A particularly advantageous polymer is a styrene-divinylbenzene copolymer, which is a cross-linked copolymer.
Other monomers can also be used as admixed components in the copolymer to be sulphonated. Such monomers include different acrylate esters and acrylic acids. Examples include methyl acrylate, ethyl acrylate, methyl methacrylate, acrylic acid, methacrylic acid and acrylonitrile, and other organic compounds with one double bond.
In addition to divinylbenzene (DVB), other possible cross-linking monomers include different known alkenylaromatic and aliphatic cross-linking agents, such as isoprene, allyl methacrylate, vinyl methacrylate, glycol dimethacrylate, glycol diacrylate and other polyunsaturated organic compounds.
The polymer to be sulphonated is preferably a gel-type polymer with a cross-linking degree of typically between 0.5 and 12%, preferably 1 and 10% of the cross-linking component, such as DVB. The polymer to be sulphonated can also be a macroporous polymer, in which case the degree of cross-linking typically varies between 4 and 30%, preferably between 8 and 20% of the cross-linking component, such as DVB.
The polymer is preferably a styrene-divinylbenzene copolymer, which can be either gel-type or macroporous. The cross-linking degree of the gel-type styrene-divinylbenzene is typically between 0.5 and 12% DVB, preferably 1 and 10% DVB. The cross-linking degree of the macroporous styrene-divinylbenzene is typically between 4 and 30% DVB, preferably between 8 and 20% DVB.
Sulphonation is carried out at a low temperature, typically from 20 to 120xc2x0 C., and preferably 40 to 80xc2x0 C.
The polymer is typically sulphonated to a sulphonation degree of 0.1 to 5.5 meq/g, preferably 0.2 to 5.5 meq/g, more preferably 0.2 to 4.2 meq/g, and most preferably 1 to 3 meq/g.
In order to obtain particles with a homogenous sulphonation degree, the particle size distribution of the polymer should preferably be as narrow as possible. The particle size distribution, given in UC units, is typically on a range of from 1 to 1.7, preferably 1 to 1.25.
The resins prepared according to the invention are typically ion-exchange resins and particularly strongly acidic cation-exchange resins.
The polymer resin according to the invention is typically sulphonated in a particle form, preferably in a spherical form. The polymer resin can also be in a fibrous form, i.e. it can consist of either staple and/or long fibres. The resin can also be in a pulverized form. Resins with a sulphonated shell layer are preferably used in a spherical form, but they can also be used in a pulverized form.
The process according to the invention employs substantially pure gaseous sulphur trioxide as the sulphonating agent. The source of sulphur trioxide can be for example pure sulphur trioxide per se. It can also be sulphur dioxide, which is oxidized in situ into sulphur trioxide. Sulphur trioxide can also be obtained from oleum (fuming sulphuric acid which contains sulphur trioxide), in which case the sulphonation is typically carried out at the vapour pressure of sulphur trioxide or at a lower pressure.
An embodiment of the invention can be implemented for example by first subjecting a reaction space, which contains the polymer used as a starting material, to a reduced pressure (e.g. about 100 Pa) in order to remove diluting gases, such as air. Substantially pure sulphur trioxide is thereafter fed into a reaction vessel containing the polymer, whereafter the sulphonation takes place at the vapour pressure of sulphur trioxide or at a lower pressure. The reaction time of the sulphonation is typically selected between 1 and 24 hours.
The sulphonation reaction is adjusted by means of the amount of sulphur trioxide used, the reaction temperature and the reaction time. The sulphonation reaction is terminated for example by dilution with air.
The sulphonation reaction proceeds according to a shrinking core mechanism, which means that the interface between the sulphonated and the unsulphonated regions is very distinct.
After the sulphonation reaction the sulphonated product is subjected to after-treatment to prevent breakage of the resin structure for example by first diluting the reaction product with sulphuric acid (50%) and thereafter washing it with water to a pH value of 5. The reaction product can also be diluted directly into water.
The process enables the preparation of a partly sulphonated product, where only the shell layer of the polymer particles has been sulphonated. Such partly sulphonated particles (less than one sulphone group per benzene ring) do not break when the sulphonated product is being diluted during the after-treatment phase. Due to their stability, resins whose shell layer has been sulphonated are useful for purposes which require resins in a spherical form. Resin particles are also useful in the production of pulverized resins.
A gel-type polymer resin prepared according to the invention has the following advantageous properties compared with a conventional resin sulphonated with sulphuric acid: it swells less than a conventional gel resin having the same cross-linking degree, it has better stability against oxidizing conditions, and corresponding thermal stability, and it has better resistance to osmotic shock, better compression resistance and a higher degree of packing. Partly sulphonated polymer resin particles prepared according to the invention do not break when the resin is being used and processed.
Macroporous resins prepared according to the invention, on the other hand, have been found to have better activity for example as catalyst resins than conventional resins sulphonated with sulphuric acid.